Having higher Speed means you will attack first in most cases. So if both Pokemon's HP is low, you will probably KO your opponent before they get a chance to hit you.

There are a few moves such as Electro Ball that inflict greater damage if you are much faster than your opponent. And moves like Payback where if you are slower and so go second in the turn, the power doubles.

i could use a priority move in that case. so if the speed is high already, i dont need to boost it?
like crobat. he is the strongest poison type. his overall stats is 535, mainly because he has great speed but low attack